The present invention relates to the separation of a multicomponent feed by distillation into at least three streams: at least one enriched in a most volatile component, at least one enriched in a least volatile component, and at least one enriched in a component of an intermediate volatility. The separation is carried out using a distillation column having a partitioned section to recover a component of intermediate volatility. The present invention also relates to the production of an argon-enriched stream from a cryogenic air separation process using a partitioned section within a primary distillation column to rectify and enrich an argon-bearing stream.
The traditional method of recovering argon from air is to use a double-column distillation system having a higher pressure column and a lower pressure column thermally linked with a reboiler/condenser and a side-arm rectifier column attached to the lower pressure column. Oxygen product is withdrawn from the bottom of the lower pressure column and at least one nitrogen-enriched stream is withdrawn from the top of the lower pressure column. Vapor provided by the reboiler of the lower pressure column rises through the bottom section of that column then splits into two portions. A first portion continues up the lower pressure column into an intermediate distillation section above. A second portion is withdrawn from the lower pressure column and passed to the side-arm column. This portion, which generally contains between 5% and 15% argon, traces of nitrogen, and the balance oxygen, is rectified in the side-arm column to produce an argon-enriched stream substantially purified of oxygen. Typically, this argon-enriched stream, commonly referred to as crude argon, is withdrawn from the top of the side-arm column with an oxygen content ranging from parts per million (ppm) levels to 3 mole %. The rectification in the side-arm column is achieved by providing liquid reflux via a condenser located at the top of the side-arm column.
Since vapor is withdrawn from the lower pressure column to feed the side-arm column, the vapor flow to the intermediate section of the lower pressure column is necessarily reduced relative to the vapor flow in the bottom section of the lower pressure column. Commonly, steps must be taken to maintain proper mass transfer performance in the intermediate section, such as reducing the diameter of the column in the intermediate section to maintain appropriate vapor velocity and/or reducing the packing density to maintain appropriate liquid loading.
In general, whenever a side rectifier or a side-stripper is employed, vapor and liquid flow rates in the intermediate distillation section of the main column (e.g., a lower pressure column) are reduced relative to the flow rates in the distillation section below and/or the distillation section above.
Divided-wall columns have been proposed in the literature as a means to better utilize a given column diameter, and thereby reduce capital cost. Divided-wall columns essentially contain multiple distillation sections at the same elevation within a single column shell. An early example of the use of a divided-wall column is disclosed in U.S. Pat. No. 2,471,134 (Wright). Wright shows how a partitioning wall may be used to produce three products from a single distillation column. In Wright, the partition forms a separation zone, the top and bottom of which communicates with the main distillation column. Divided-wall columns of the type disclosed by Wright are discussed further by Lestak and Collins in "Advanced Distillation Saves Energy and Capital", Chemical Engineering, pages 72-76, July 1997. Christiansen, Skogestad, and Lien disclose further applications for divided-wall columns in "Partitioned Petlyuk Arrangements for Quaternary Separations", Distillation and Absorption '97, Institution of Chemical Engineers, Symposium Series No. 142, pages 745-756, 1997.
In "Multicomponent Distillation--Theory and Practice", by Petluyuk and Cerifimow (page 198, figure VI-4e, published by Moscow Chemie, 1983) the authors disclose a configuration for a divided-wall column where the partitioning wall is cylindrical and forms an annular separation zone, the top and bottom of which communicates with the main distillation column.
U.S. Pat. No. 5,946,942 (Wong, et al.) discloses an application of divided-wall principles to air separation. Wong discloses an apparatus wherein the lower pressure column contains an inner annular wall. The region contained between the inner annular wall and the outer shell of the lower pressure column constitutes a section for the production of argon product. A drawback of this divided-wall column for argon recovery stems from the geometry of the device used, as explained below.
The cross sectional geometry of the argon rectification section taught by Wong is annular. At the top of the annular section, the rising vapor must be collected and withdrawn. If a single outlet pipe is used, vapor from the farthest location in the annulus must travel significantly farther than vapor from the nearest location. This introduces flow maldistribution of vapor within the separation section below. Similarly, maldistribution of liquid also is a concern, especially if the separation section below uses packing. It is possible to mitigate maldistribution by taking steps, such as using multiple outlet and inlet pipes, but the result is a more complex and costly design. Furthermore, use of an annular geometry produces a relatively large wall surface area. Large wall surface area is discouraged when packing is used, because liquid tends to migrate to the walls, thereby introducing liquid flow maldistribution.
It is desired to have a process using the divided-wall concept which minimizes vapor and liquid maldistribution in the argon section of a distillation column.
It is further desired to have a process using the divided-wall concept which minimizes vapor and liquid maldistribution in any partitioned section used to recover a component enriched in an intermediate-volatility component.
It also is desired to have a process for separation of a multicomponent fluid which overcomes the difficulties and disadvantages of the prior art to provide better and more advantageous results.